Pneumatic tire

ABSTRACT

Disclosed is a pneumatic tire which is allowed to increase block stiffness not only during braking and driving but also during cornering, and is allowed thereby to compatibly enhance tire performances both during braking and driving and during cornering. The pneumatic tire of the present invention is one where a plurality of longitudinal grooves extending in a tire circumferential direction and a plurality of lateral grooves extending in a tire widthwise direction are provided in a tread portion, a plurality of blocks are defined by these longitudinal and lateral grooves, and a plurality of sipes extending in the tire widthwise direction are provided to each of the blocks, wherein, with regard to each of the sipes, a zigzag shape is formed on a tread surface, bent portions ranging in the tire widthwise direction while bent in the tire circumferential direction are formed inside the block at at least two positions in the tire radial direction, and a zigzag shape with an amplitude in the tire radial direction is formed in each of the bent portions.

TECHNICAL FIELD

The present invention relates to a pneumatic tire where a plurality ofsipes are provided in a block, and more specifically, relates to apneumatic tire which is allowed, by shapes of the sipes, to increaseblock stiffness not only during braking and driving but also duringcornering, and is allowed thereby to compatibly enhance tireperformances both during braking and driving and during cornering.

BACKGROU D ART

In an icy and snowy road pneumatic tire, as improving measures foron-ice performances, increasing an edge amount of sipes provided in ablock thereof, reducing hardness of tread rubber, and the like have beengenerally attempted. However, in the case where tread rubber is reducedin hardness, block stiffness is reduced. This causes the block to falldown during braking and driving and during cornering, and hence leads toreduction in ground contact area of the tire, and as a result, tireperformances both during summer and during winter are deteriorated.Accordingly, for the purpose of preventing the block from falling down,it has been proposed that a sipe should be formed in a three-dimensionalshape.

As a sipe having a three-dimensional shape, one where a zigzag shape isformed on a tread surface, and an amplitude of the zigzag shape isvaried inside the block has been proposed (for example, refer to PatentDocument 1). In this case, although it is possible to increase blockstiffness during braking and driving, there is a disadvantage that aneffect of increasing block stiffness during cornering can be scarcelyobtained.

In addition, a sipe where a zigzag shape is formed on a tread surface,and triangular pyramids and inverted triangular pyramids are alternatelyarranged inside the block has been proposed (for example, refer toPatent Document 2). In this case, although it is possible to expect aneffect of increasing block stiffness during cornering, there is adisadvantage that a place where it can be arranged is limited becausethe sipe has a direction.

Moreover, a sipe where a zigzag shape is formed on a tread surface, andbent portions ranging in a tire radial direction and being bent in atire widthwise direction are formed inside the block, has been proposed(for example, refer to Patent Document 3). In this case as well, thereis a disadvantage that block stiffness during cornering is low ascompared to block stiffness during braking and driving.

[Patent Document 1] Japanese patent application Kokai publication No.2000-6619

[Patent Document 2] Japanese patent application Kokai publication No.2002-301910

[Patent Document 3] Japanese patent application Kokai publication No.2002-321509

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide a pneumatic tire whichis allowed, depending on a shape of sipes, to increase block stiffnessnot only during braking and driving but also during cornering, and isallowed thereby to compatibly enhance tire performances both duringbraking and driving and during cornering.

A pneumatic tire for achieving the above object is a pneumatic tirewhere a plurality of longitudinal grooves extending in a tirecircumferential direction and a plurality of lateral grooves extendingin a tire widthwise direction are provided in a tread portion, aplurality of blocks are defined by these longitudinal and lateralgrooves, and a plurality of sipes extending in the tire widthwisedirection are provided to each of the blocks. The tire is characterizedin that, with regard to each of the sipes, a zigzag shape with anamplitude in the tire circumferential direction is formed on a treadsurface, bent portions ranging in the tire widthwise direction whilebent in the tire circumferential direction are formed inside the blockat at least two positions in a tire radial direction, and a zigzag shapewith an amplitude in the tire radial direction is formed in each of thebent portions.

In the present invention, because each of the sipes is provided in atleast two positions in the tire radial direction with the bent portionsbent in the tire circumferential direction and ranging in the tirewidthwise direction, small blocks on both sides of the sipe are engagedwith each other to suppress deformation of a block even during brakingand driving, whereby tire performances during braking and driving can beenhanced. In addition, with regard to the above-described sipe, a zigzagshape with an amplitude in the tire radial direction is formed in thebent portions, whereby small blocks on both sides of the sipe areengaged with each other to suppress deformation of a block even duringcornering, whereby tire performances during cornering can be enhanced.Therefore, even if tread rubber is reduced in hardness, tireperformances during braking and driving and those during cornering arecompatibly enhanced. Additionally, because the above sipe has nodirectionality, a place where it is arranged is not limited.Furthermore, according to the above sipe, there is an advantage that adifference is small between edge lengths when the tire is new and thatwhen the tire is in a middle period and later in terms of abrasion.

In the present invention, in order to sufficiently obtain, withoutdamaging mold releasability, an improving effect on tire performancesboth during braking and driving and during cornering, it is preferablethat a tilt angle of the sipe in the tire circumferential direction to anormal-line direction of the tread surface be in a range of 10 to 45degrees, and the amplitude of the bent portion of the sipe in the tireradial direction be in a range of 0.5 to 5.0 mm.

According to the present invention, it is possible to suppressdeterioration in mold releasability by appropriately setting the tiltangle of the sipe in the tire circumferential direction to thenormal-line direction of the tread surface and the amplitude of the bentportion of the sipe in the tire radial direction. In some cases,however, there arises a failure such as a breakage in the blockgenerated by a sipe-forming blade used during mold release.

Consequently, for the purpose of more securely avoiding such a failureas the block break, in the above pneumatic tire, it is preferable that,while the amplitude of the sipe in the tire circumferential direction beconstant, a tilt angle of the sipe in the circumferential direction to anormal-line direction of a tread surface be smaller in a portion closerto a bottom of the sipe than that in a portion closer to the treadsurface, and the amplitude of the bent portions in the tire radialdirection be larger in a portion closer to a bottom of the sipe thanthat in a portion closer to the tread surface.

That is, even in the case where the amplitude of the sipe in the tirecircumferential direction is constant, releasability from a mold can beenhanced by setting a tilt angle of the sipe in the tire circumferentialdirection to the normal-line direction of the tread surface to besmaller in a portion closer to the sipe bottom. Moreover, even in thecase where tilt angles of the sipe in the tire circumferential directionare thus varied, reduction in block stiffness during braking and drivingis suppressed, and at the same time, block stiffness during cornering isincreased by having the amplitude of the bent portions in the tireradial direction to be larger in the portion closer to the sipe bottom.

In this case, it is preferable that tilt angles of the sipe in the tirecircumferential direction to the normal-line direction of the treadsurface be between 30 and 45 degrees in the portion closest to the treadsurface, and be not smaller than 15 degrees, but smaller than 30 degreesin the portion closest to the sipe bottom. Moreover, it is preferablethat amplitudes of the bent portions in the tire radial direction be notless than 0.5 mm in the portion closest to the tread surface, and is setnot more than 3.5 mm in the portion closest to the sipe bottom.

Furthermore, for the purpose of more securely avoiding such a failure asthe block break, in the above pneumatic tire, it is desirable that,while intervals between the bent portions of the sipe in the tire radialdirection be uniform, the amplitude in the tire circumferentialdirection be smaller in a portion closer to the sipe bottom. Thereby,resistance during mold release is reduced, and therefore releasabilityof the pneumatic tire from a mold can be enhanced.

In this case, it is preferable to employ the following structure for thepurpose of ensuring favorable releasability. Specifically, if it isassumed that there are a reference line passing through a positionrepresenting an outer limit of the amplitude of the sipe on the treadsurface and extending in the normal-line direction of the tread surface,and an auxiliary line defining, along the depth direction of the sipe,the amplitude of the sipe in the tire circumferential direction, it ispreferable that a distance at the sipe bottom between the reference lineand the auxiliary line be more than 0% and not more than 50% of theamplitude of the sipe in the tire circumferential direction on the treadsurface.

Alternatively, if it is assumed that there are a reference line passingthrough a position representing an outer limit of an amplitude of thesipe on the tread surface and extending in the normal-line direction ofthe tread surface, an upper auxiliary line defining the amplitude of thesipe in the tire circumferential direction in the upper portion of thesipe, and a lower auxiliary line defining the amplitude of the sipe inthe tire circumferential direction in the lower portion of the sipe, itis preferable that tilt angles respectively of the upper and lowerauxiliary lines to the reference line be made different from each other,and the tilt angle of the lower auxiliary line be larger than that ofthe upper auxiliary line. More specifically, it is preferable that adistance at the sipe bottom between the reference line and the upperauxiliary line be more than 0% and not more than 25% of the amplitude ofthe sipe in the tire circumferential direction on the tread surface, anda distance at the sipe bottom between the reference line and the upperauxiliary line be more than 25% and not more than 50% of the amplitudeof the sipe in the tire circumferential direction on the tread surface.

In the present invention, for the purpose of allowing excellenttraveling performances to be exerted on icy and snowy roads immediatelyafter the start of use, it is possible to provide, on a superficialportion of a block, a plurality of shallow grooves whose depth is in arange of 0.1 to 1.0 mm and which are shallower than the sipes. In apneumatic tire to which the thus described shallow grooves are provided,it is preferable that a vertical portion extending in a normal-linedirection of the tread surface be provided to the sipe in a sectionwhere the sipe joins to the tread surface, for the purpose of avoiding afailure in mold release which occurs due to interference between theshallow groove and the sipe. It is favorable that a height of thevertical portion of the sipe be not less than the depth of the shallowgroove.

Although remarkable operational effects can be obtained when the presentinvention is applied to an icy and snowy road pneumatic tire representedby a studless tire, it is also possible to apply the invention to aall-season pneumatic tire.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view showing a tread pattern of an icy and snowy roadpneumatic tire formed of an embodiment of the present invention.

FIG. 2 is a partially crosscut perspective view showing a block of theicy and snowy road pneumatic tire formed of the embodiment of thepresent invention.

FIG. 3 is a side view of the block in FIG. 2.

FIG. 4 is a side view showing an inner wall surface of a sipe in theblock in FIG. 2.

FIG. 5 is a partially crosscut perspective view showing a block of anicy and snowy road pneumatic tire formed of a first other embodiment ofthe present invention.

FIG. 6 is a side view showing an inner wall surface of a sipe in theblock in FIG. 5.

FIG. 7 is a cross-sectional view taken along a VII-VII line of FIG. 6.

FIGS. 8 a to 8 c show parts of an inner side wall of the sipe in theblock of the icy and snowy road pneumatic tire formed of a second otherembodiment of the present invention, and FIGS. 8 a, 8 b, and 8 c are aside view, a cross-sectional view taken along an VIII-VIII line of FIG.8 a, and a cross-sectional view taken along an VIII′-VIII′ line of FIG.8 a, respectively.

FIGS. 9 a to 9 c show parts of an inner side wall of a sipe in a blockof an icy and snowy road pneumatic tire formed of a third otherembodiment of the present invention, and FIGS. 9 a, 9 b, and 9 c are aside view, a cross-sectional view taken along a IX-IX line of FIG. 9 a,and a cross-sectional view taken along a IX′-IX′ line of FIG. 9 a,respectively.

FIG. 10 is a side view showing a block of an icy and snowy roadpneumatic tire formed of a fourth other embodiment of the presentinvention.

FIGS. 11 a to 11 c show parts of an inner side wall of a sipe in theblock in FIG. 10, and FIGS. 11 a, 11 b, and 11 c are a side view, across-sectional view taken along an XI-XI line of FIG. 11 a, and across-sectional view taken along an XI′-XI′ line of FIG. 11 a,respectively.

FIG. 12 is a perspective view showing a block of Conventional Example 1.

FIG. 13 is a perspective view showing a block of Conventional Example 2.

FIG. 14 is a perspective view showing a block of Conventional Example 3.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, detailed descriptions will be given of configurations ofthe present invention with reference to the attached drawings.

FIG. 1 shows a tread pattern of an icy and snowy road pneumatic tireformed of an embodiment of the present invention, and FIG. 2 shows ablock thereof. FIG. 3 shows a side of the above block, and FIG. 4 showsan inner wall surface of a sipe in the above block.

As shown in FIG. 1, a plurality of longitudinal grooves 2 extending in atire circumferential direction, and a plurality of lateral grooves 3extending in a tire widthwise direction are formed on a tread portion 1,and a plurality of blocks 4 are defined by theses longitudinal grooves 2and lateral grooves 3. Additionally, in each of the blocks 4, aplurality of sipes 5 extending in the tire widthwise direction areformed. Note that a shape of the block 4 and the number of sipes 5 arenot particularly limited.

As shown in FIG. 2, each of the sipes 5 forms a zigzag shape having itsamplitude in the tire circumferential direction on a tread surface S.Inside the block, in at least two positions in a tire radial direction(Tr), a plurality of bent portions 6 bent in the tire circumferentialdirection (Tc) and ranging in the tire width direction (Tw) are formed.These bent portions 6 include a convex bent portion 6 a and a concavebent portion 6 b. On one wall surface of the sipe 5, the convex bentportion 6 a and the concave bent portion 6 b are alternately arranged,and on the other wall surface (not shown) facing the one wall surface, apositional relation with respect to the convex bent portion 6 a and theconcave bent portion 6 b is opposite to that on the one wall surface. Inthe case where the bent portions 6 bent in the tire circumferentialdirection are provided in each of the sipes 5, small blocks on bothsides of each of the sipes 5 are engaged with each other during brakingand driving, whereby deformation of the black 4 is suppressed, and hencefalling down of the block 4 in the tire circumferential direction can besuppressed. Note that, by providing at least two of the bent portions 6with respect to each of the sipes 5, a difference in block stiffnessoccurring from a forward rotation and a backward rotation can beavoided.

As shown in FIG. 3, a tilt angle θ of the sipe 5 in the tirecircumferential direction to a normal-line direction of the treadsurface S may be set in a range of 10 to 45 degrees. If the tilt angle θis less than 10 degrees, an effect of bearing the falling down of theblock 4 during braking and driving becomes insufficient, and to thecontrary, if the tilt angle θ exceeds 45 degrees, mold releasability isdeteriorated.

As shown in FIG. 4, in the bent portions 6, the sipe 5 forms a zigzagshape with an amplitude T in the tire radial direction (Tr). In the casewhere, in the bent portions 6, the sipe 5 is formed in a zigzag shapewith an amplitude T in the tire radial direction, small blocks on bothsides of each of the sipes 5 are engaged with each other duringcornering, whereby deformation of the black 4 is suppressed, and hencefalling down of the block 4 in the tire widthwise direction issuppressed. It is favorable that the amplitude T of the bent portions 6be set in a range of 0.5 to 5.0 mm. If this amplitude T is less than 0.5mm, an effect of bearing the falling down of the block 4 duringcornering becomes insufficient, and to the contrary, if it exceeds 5.0mm, mold releasability is deteriorated.

In the above-described icy and snowy road pneumatic tire, it isfavorable that a JIS-A hardness (at 0° C.) of a rubber compositionconstituting the tread portion be in a range of 40 to 60, and preferably45 to 55. If the JIS-A hardness of the tread rubber is less than 40, thefalling down of the block 4 becomes more likely to occur, and to thecontrary, if it exceeds 60, on-ice friction is reduced.

According to the above-described icy and snowy road pneumatic tire,since the sipe 5 is provided with bent portions 6 bent in the tirecircumferential direction (Tc) and ranging in the tire width direction(Tw) in at least two positions in the tire radial direction (Tr), smallblocks on both sides of the sipe 5 are engaged with each other duringbraking and driving, whereby the deformation of the block 4 issuppressed, and hence tire performances during braking and driving canbe enhanced. Moreover, since, with regard to the sipe 5, a zigzag shapewith an amplitude T in the tire radial direction (Tr) is formed in thebent portions 6, small blocks on both sides of the sipe 5 are engagedwith each other also during cornering, whereby block deterioration issuppressed, and hence tire performances during cornering can beenhanced.

Accordingly, even if where tread rubber is reduced in hardness, itbecomes possible to compatibly enhance tire performances during brakingand driving and tire performances during cornering. Particularly,because it becomes possible to increase a number of sipes per block andto use rubber which is low in hardness as tread rubber, tireperformances during summer can be maintained while on-ice performancesis enhanced.

FIG. 5 shows a block of an icy and snowy road pneumatic tire formed of afirst other embodiment of the present invention. Additionally, FIG. 6shows parts of an inner wall surface of a sipe in the block in FIG. 5,and FIG. 7 is a cross-sectional view taken along a VII-VII line of FIG.6.

In FIG. 5, on a tread surface S, each of the sipes 5 forms a zigzagshape with an amplitude in the tire circumferential direction. Insidethe block, each of the sipes 5 forms in at least two positions in a tireradial direction (Tr) a plurality of bent portions 6 bent in the tirecircumferential direction (Tc) and ranging in the tire width direction(Tw). These bent portions 6 include a convex bent portion 6 a and aconcave bent portion 6 b. On one wall surface of the sipe 5, the convexbent portion 6 a and the concave bent portion 6 b are alternatelyarranged, and on the other wall surface (not shown) facing the one wallsurface, a positional relation with respect to the convex bent portion 6a and the concave bent portion 6 b is opposite to that on the one wallsurface. In the case where the bent portions 6 bent in the tirecircumferential direction are provided in each of the sipes 5, smallblocks on both sides of each of the sipes 5 are engaged with each otherduring braking and driving, whereby deformation of the block 4 issuppressed, and hence falling down of the block 4 in the tirecircumferential direction is suppressed. Note that, by providing atleast two of the bent portions 6 with respect to each of the sipes 5, adifference in block stiffness occurring from forward and backwardrotations of the tire can be avoided.

As shown in FIG. 6, in the bent portions 6, the sipe 5 forms zigzagshapes which have amplitudes A, B, C and D, respectively, in the tireradial direction (Tr). In the case where the sipe 5 forms, in the bentportions 6, zigzag shapes which have amplitudes A, B, C and D,respectively, in the tire radial direction (Tr), small blocks on bothsides of the sipe 5 are engaged with each other during cornering,whereby deformation of the block 4 is suppressed, and hence the fallingdown of the block 4 in the tire widthwise direction can be suppressed.

In FIG. 6, the amplitudes A, B, C and D of the bent portions 6 in thetire radial direction are larger in portions closer to the sipe bottomthan in portions closer to the tread surface. In a manner that theamplitudes A, B, C and D of the bent portions 6 in the tire radialdirection are made larger in the portions closer to the sipe bottom,while reduction in block stiffness during braking and driving can besuppressed, block stiffness during cornering also can be enhanced.

More specifically, it is favorable that the amplitude A of the bentportions 6 at the portion closest to the tread surface be set to atleast 0.5 mm, and that the amplitude D of the bent portions 6 at theportion closest to the sipe bottom be set to 3.5 mm or less. If theamplitude A of the bent portions 6 at the portion closest to the treadsurface is less than 0.5 mm, reduction in block stiffness becomesremarkable. On the other hand, if the amplitude D of the bent portions 6at the portion closest to the sipe bottom exceeds 3.5 mm, mold releasebecomes difficult. By forming the sipe 5 in a manner that the amplitudesA, B, C and D of the bent portions 6 are gradually increased from theportion closest to the tread surface to the portion closest to the sipebottom, it becomes possible to compatibly achieve maintenance of blockstiffness and mold releasability in a balanced manner.

As shown in FIG. 7, an amplitude X of the sipe 5 in the tirecircumferential direction is set constant over an entire length in adepth-wise direction of the sipe 5. Additionally, tilt angles a, b, cand d of the sipe 5 to a normal-line direction of the tread surface Sare smaller in portions closer to the sipe bottom than in portionscloser to the tread surface. In a manner that the tilt angles a, b, cand d of the sipe 5 to a normal-line direction of the tread surface Sare decreased from the portion closest to the tread surface to theportion closest to the sipe bottom, mold releasability can be enhanced.

More specifically, it is favorable that the tilt angles a at the portionclosest to the tread surface be set between 30 degrees and 40 degrees,and that the tilt angles d at the portion closest to the sipe bottom beset not smaller than 15 degrees, but smaller than 30 degrees. If thetilt angles a at the portion closest to the tread surface is less than30 degrees, reduction in block stiffness becomes remarkable, and to thecontrary, if it exceeds 45 degrees, mold release becomes difficult. Ifthe tilt angles d at the portion closest to the sipe bottom is less than15 degrees, reduction in block stiffness becomes remarkable, and to thecontrary, if it is not less than 30 degrees, mold release becomesdifficult. By forming the sipe 5 in a manner that the tilt angles a, b,c and d are gradually decreased from the portion closest to the treadsurface to the portion closest to the sipe bottom, more smooth moldrelease becomes possible.

According to the above-described icy and snowy road pneumatic tire, asin the case with the foregoing embodiment, even if tread rubber isreduced in hardness, it is possible to compatibly enhance tireperformances during braking and driving and tire performances duringcornering. Moreover, according to the present embodiment, moldreleasability is enhanced, whereby a failure such as a breakage in theblock can be more securely avoided.

FIGS. 8 a to 8 c show parts of an inner side wall of the sipe in theblock of the icy and snowy road pneumatic tire formed of a second otherembodiment of the present invention, and FIGS. 8 a, 8 b, and 8 c are aside view, a cross-sectional view taken along an VIII-VIII line of FIG.8 a, and a cross-sectional view taken along an VIII′-VIII′ line of FIG.8 a, respectively. In the present embodiment, a basic structure of theblock is the same as that shown in FIGS. 1 and 2, and a detaileddescription thereof will be omitted.

In FIGS. 8 a to 8 c, the sipe 5 is formed in a manner that: intervals λbetween bent portions 6 including the convex bent portions 6 a andconcave bent portions 6 b in the tire radial direction (Tr) are madeconstant; and amplitudes of the bent portions 6 in the tirecircumferential direction (Tc) are gradually decreased with increasingcloseness to the sipe bottom. Here, that the intervals λ are constantmeans that a range of variation among the internals λ is not more than1.0 mm.

If it is assumed that there are a reference line L (a straight line)passing through a position representing an outer limit of an amplitudeof the sipe 5 on a tread surface S and extending in the normal-linedirection of the tread surface S, and an auxiliary line G (a straightline) defining, along the depth direction of the sipe, the amplitude ofthe sipe 5 in the tire circumferential direction (Tc), it is preferablethat a distance W at the sipe bottom between the reference line L andthe auxiliary line G be set more than 0% and not more than 50% of theamplitude X of the sipe 5 in the tire circumferential direction on thetread surface S. That is, it is favorable that a relation expressed by 0mm<W≦(½)X be satisfied. If this distance W does not exceed 0 mm, animproving effect on mold releasability cannot be obtained, and to thecontrary, if it exceeds 50% of the amplitude X, block stiffness becomesinsufficient.

According to the above-described icy and snowy road pneumatic tire, asin the case with the foregoing embodiments, even if tread rubber isreduced in hardness, it is possible to compatibly enhance tireperformances during braking and driving and tire performances duringcornering. Moreover, according to the present embodiment, moldreleasability is enhanced, whereby a failure such as a breakage in theblock can be more securely avoided. Note that, if the intervals λbetween the bent portions 6 in the tire radial direction (Tr) areincreased with increasing closeness to the bottom of the sipe 5,although an improving effect on mold releasability can be expected, itis difficult to apply such a structure to a case using shallow sipes.Alternatively, if the amplitudes of the bent portions 6 in the tirecircumferential direction (Tc) are decreased with increasing closenessto the bottom of the sipe 5 while the intervals λ between the bentportions 6 in the tire radial direction (Tr) are made constant, animproving effect on mold releasability also can be securely enjoyed inthe case of using shallow sipes.

FIGS. 9 a to 9 c show parts of an inner side wall of a sipe in a blockof an icy and snowy road pneumatic tire formed of a third otherembodiment of the present invention, and FIGS. 9 a, 9 b, and 9 c are aside view, a cross-sectional view taken along a IX-IX line of FIG. 9 a,and a cross-sectional view taken along a IX′-IX′ line of FIG. 9 a,respectively. In the present embodiment, a basic structure of the blockis the same as that shown in FIGS. 1 and 2, and a detailed descriptionthereof will be omitted.

In FIGS. 9 a to 9 c, it is assumed that there are a reference line L (astraight line) passing through a position representing an outer limit ofan amplitude of the sipe 5 on a tread surface S and extending in thenormal-line direction of the tread surface S, an upper auxiliary line G1(a straight line) defining an amplitude of the sipe 5 in the tirecircumferential direction (Tc) in the upper portion Y1 of the sipe 5,and a lower auxiliary line G2 (a straight line) defining an amplitude ofthe sipe 5 in the tire circumferential direction (Tc) in the lowerportion Y2 of the sipe 5. Under that assumption, tilt anglesrespectively of the upper and lower auxiliary lines G1 and G2 to thereference line L are made different from each other, and the tilt angleof the lower auxiliary line G2 is larger than that of the upperauxiliary line G1.

In particular, it is preferable that: a distance W1 at the bottom of thesipe 5 between the reference line L and the upper auxiliary line G1 bemore than 0% and not more than 25% of the amplitude X of the sipe 5 onthe tread surface S in the tire circumferential direction (Tc); and adistance W2 at the bottom of the sipe 5 between the reference line L andthe upper auxiliary line G2 be not less than 25% and not more than 50%of the amplitude X of the sipe 5 on the tread surface S in the tirecircumferential direction (Tc). That is, it is favorable that relationsrespectively expressed by 0 mm<W1≦(¼)X, and (¼)X≦W2≦(½)X be satisfied.

According to the above-described icy and snowy road pneumatic tire, asin the case with the foregoing embodiments, even if tread rubber isreduced in hardness, it is possible to compatibly enhance tireperformances during braking and driving and tire performances duringcornering. Moreover, according to the present embodiment, whilereduction in block stiffness is suppressed to a minimum level by makingrelatively small the tilt angle of the auxiliary line G1 in the upperportion Y1 of the sipe 5, an improving effect on mold releasability canbe fully exerted by making relatively large the tilt angle of theauxiliary line G2 in the lower portion Y2 of the sipe 5.

FIG. 10 shows a block of an icy and snowy road pneumatic tire formed ofa fourth other embodiment of the present invention. FIGS. 11 a to 11 cshow parts of an inner side wall of a sipe in the foregoing block, andFIGS. 11 a, 11 b, and 11 c are a side view, a cross-sectional view takenalong an XI-XI line of FIG. 11 a, and a cross-sectional view taken alongan XI′-XI′ line of FIG. 11 a, respectively. In the present embodiment, abasic structure of the block is the same as that shown in FIGS. 1 and 2,and a detailed description thereof will be omitted.

In FIG. 10, for the purpose of allowing excellent traveling performancesto be exerted on icy and snowy roads immediately after the start of use,a plurality of shallow grooves 7 which are shallower than the sipes 5are formed on a superficial portion of the block 4. A depth D1 of eachof the shallow grooves 7 is set in a range of 0.1 to 1.0 mm. Anextending direction of these shallow grooves 7 is not particularlylimited, and for example, they can be arranged in a manner that theytilt with respect to the tire circumferential direction.

In the case where such shallow grooves 7 as described above areprovided, it is favorable, for the purpose of avoiding a failure in moldrelease which occurs due to interference between the shallow groove 7and the sipe 5, that a vertical portion extending in a normal-linedirection of the tread surface S be provided in a section of the sipe 5joining to a tread surface S as shown in FIGS. 11 a to 11 c.Additionally, it is preferable that a height D2 of the vertical portionof the sipe 5 be equal to or greater than the depth D1 of the shallowgroove 7. That is, it is favorable that a relation expressed by D1≦D2 besatisfied. With these configurations, interference on the tread surfaceS between the minute shallow groove 7 (surface work) and the sipe 5 isprevented, whereby mold releasability can be further enhanced.

Note that, although amplitudes of the sipe 5 in the tire circumferentialdirection (Tc) are defined based on a single auxiliary line G in theembodiment shown in FIGS. 11 a to 11 c, the amplitudes may be defined bya combination of an upper auxiliary line G1 and a lower auxiliary lineG2 as in the case with the preceding embodiment.

While the detailed descriptions have been given of the preferredembodiments of the present invention hereinabove, it should beunderstood that various modifications to, substitutions for, andreplacements with the preferred embodiment can be carried out as long asthe modifications, the substitutions, and the replacements do not departfrom the spirit and the scope of the present invention defined by theattached claims.

EXAMPLES

First, tires for Conventional Examples 1 to 3 and Example 1 wereprepared as icy and snowy road pneumatic tires each of which had a tiresize of 195/65R15 91Q and a block pattern, and which were made variouslydifferent only in shape of the sipes provided in the block.

Conventional Example 1 is the one, as described in Japanese patentapplication Kokai publication No. 2000-6619, adopting sipes each ofwhich forms a zigzag shape on a tread surface of the tire, and hasamplitudes of zigzag shapes varied inside the block (refer to FIG. 12).Conventional Example 2 is the one, as described in Japanese patentapplication Kokai publication No. 2002-301910, adopting sipes each ofwhich forms a zigzag shape on a tread surface of the tire, and hastriangular pyramids and inverted triangular pyramids alternatelyarranged inside the block (refer to FIG. 13). Conventional Example 3 isthe one, as described in Japanese patent application Kokai publicationNo. 2002-321509, adopting sipes each of which forms a zigzag shape on atread surface of the tire, and inside the block, has bent portions bentin a tire widthwise direction and ranging in a tire radial direction(refer to FIG. 14). On the other hand, Example 1 is the one adoptingsipes in FIG. 2.

For these test tires, on-ice braking performances, wet brakingperformances, and wet cornering performances were respectively assessedby using the following test methods, and results thereof are shown inTable 1.

Wet Braking Performance:

Each of the test tires was mounted onto a front-engine rear-drivevehicle having a displacement of 2000 cc, with conditions being a rimsize of 15×6.5JJ and an air pressure of 200 kPa. Then, braking wasperformed from a traveling state with 40 km/h on a frozen road, and abraking distance thereof was measured. The assessment results wererepresented in index numbers, which were obtained by using inverses ofmeasured values and presuming the inverse for Conventional Example 1 tobe 100. The larger index number means that the tire is more excellent inon-ice braking performance.

Wet Braking Performance:

Each of the test tires was mounted onto a front-engine rear-drivevehicle having a displacement of 2000 cc, with conditions being a rimsize of 15×6.5JJ and an air pressure of 200 kPa. Then, braking wasperformed from a traveling state with 100 km/h on a wet road with awater depth of 1 mm, and a braking distance thereof was measured. Theassessment results were represented in index numbers, which wereobtained by using inverses of measured values and presuming the inversefor Conventional Example 1 to be 100. The larger index number means thatthe tire is more excellent in wet braking performance.

Wet Cornering Performance:

Each of the test tires was mounted onto a front-engine rear-drivevehicle having a displacement of 2000 cc, with conditions being a rimsize of 15×6.5JJ and an air pressure of 200 kPa. Then, steady-statecircular cornering with a radius of 30 m was performed on a wet roadwith a water depth of 1 mm, and a maximum lateral acceleration thereofwas measured. The assessment results were represented in index numbersobtained by presuming a measured value for Conventional Example 1 to be100. The larger index number means that the tire is more excellent inwet cornering performance. TABLE 1 Conven- Conven- Conven- tional tionaltional Example 1 Example 2 Example 3 Example 1 Sipe shape (drawing) SipeNot Present Not Not directionality present present present On-icebraking 100 103 95 103 performance Wet braking 100 103 95 103performance Wet cornering 100 105 105 110 performance

As is found from this Table 1, the tire for Example 1 was more excellentthan those in Conventional Examples 1 to 3 in on-ice brakingperformance, wet braking performance, and wet cornering performance.

Next, tires for Examples 2 to 8 were prepared, by using the tire forExample 1 as reference, in a manner that the tires were made variouslydifferent in tilt angles a, b, c and d of the sipe and in amplitudes A,B, C and D of bent portions of the sipe compared with that in thereference.

For these test tires, on-ice braking performances were assessed, andfailure incidences during vulcanization were also found. Results thereofare shown in Table 2. Note that the on-ice braking performances wererepresented in index numbers by presuming a value for Example 1 to be100. The failure incidence is a percentage (%) of failed tires relativeto a number of vulcanized tires when occurrence of failures such as abreak and a cut damage generated by a sipe forming blade was inspectedfor vulcanized tires. TABLE 2 Example Example Example Example ExampleExample Example Example 1 2 3 4 5 6 7 8 Tilt angle a (degrees) 35 40 3045 40 40 40 40 Tilt angle b (degrees) 35 35 30 38 32 36 35 35 Tilt anglec (degrees) 35 30 25 32 23 33 30 30 Tilt angle d (degrees) 35 25 25 2515 29 25 25 Amplitude A (mm) 1.2 1.2 1.2 1.2 1.2 1.2 0.5 0.5 Amplitude B(mm) 1.2 1.6 1.6 1.6 1.6 1.6 1.2 1.5 Amplitude C (mm) 1.2 2.1 2.1 2.12.1 2.1 1.9 2.5 Amplitude D (mm) 1.2 2.5 2.5 2.5 2.5 2.5 2.5 3.5 On-icebraking performance 100 104 99 106 100 105 98 103 Failure incidence (%)18 0 0 0 0 0 0 0

As is found from this Table 2, the tires for Examples 2 to 8 were withextremely low failure incidences while they had equivalent levels inon-ice braking performance to that of Example 1.

Next, tires for Examples 11 to 14 were prepared as icy and snowy roadpneumatic tires each of which had a tire size of 195/65R15 91Q and ablock pattern, and which were made variously different only in shape ofthe sipes provided in the block.

In each of these Examples 11 to 14, adopted were sipes in each of whicha zigzag shape with an amplitude in the tire circumferential directionon the tread surface is formed, bent portions bent in a tirecircumferential direction and ranging in a tire widthwise direction atat least two positions in the tire radial direction is formed inside ablock, and a zigzag shape with an amplitude in the tire radial directionis formed in each of the bent portions. Additionally, in each of thesipes, intervals between the bent portions in the tire radial directionare constant, and amplitudes of the bent portions in the tirecircumferential direction are smaller in a portion closer to the sipebottom (refer to FIGS. 8 a to 8 c and FIGS. 9 a to 9 c). Note that, ineach of Examples 13 and 14, shallow grooves with a depth of 0.3 mm wereprovided in a superficial portion of the block, and a vertical portionextending in a normal-line direction of the tread surface was providedin a section where the sipe joins to the tread surface.

For these test tires, by using the above-described test methods, on-icebraking performances, wet braking performances, and wet corneringperformances were respectively assessed, and failure incidences duringvulcanization were also found. Results thereof are shown in Table 3. InTable 3, results for Conventional Examples 1 to 3 described above arealso shown. TABLE 3 Conventional Conventional Conventional ExampleExample Example Example Example 1 Example 2 Example 3 11 12 13 14Presence of vertical Not present Not present Not present Not Not PresentPresent portion of sipe present present Tilting of sipe Not present Notpresent Not present Constant Not Constant Not auxiliary line constantconstant Distance W of — — — (¼)× — (¼)× — auxiliary line Distance W1 of— — — — ( 1/10)× — ( 1/10)× auxiliary line Distance W2 of — — — — (¼)× —(¼)× auxiliary line On-ice braking 100 103 95 103 107 104 108performance Wet braking 100 103 95 103 107 104 108 performance Wetcornering 100 105 105 110 110 110 110 performance Failure incidence (%)10 20 15 2 2 0 0

As is found from this Table 3, the tires for Examples 11 to 14 were moreexcellent than those for Conventional Examples 1 to 3 in on-ice brakingperformance, wet braking performance, and wet cornering performance.Furthermore, all of the tires for Examples 11 to 14 were with extremelylow failure incidences.

1. A pneumatic tire where a plurality of longitudinal grooves extendingin a tire circumferential direction and a plurality of lateral groovesextending in a tire widthwise direction are provided in a tread portion,a plurality of blocks are defined by these longitudinal and lateralgrooves, and a plurality of sipes extending in the tire widthwisedirection are provided to each of the blocks, wherein, with regard toeach of the sipes, a zigzag shape with an amplitude in the tirecircumferential direction is formed on a tread surface, bent portionsranging in the tire widthwise direction while bent in the tirecircumferential direction are formed inside the block at at least twopositions on a tire radial direction, and a zigzag shape with anamplitude in the tire radial direction is formed in each of the bentportions.
 2. The pneumatic tire according to claim 1, wherein a tiltangle of the sipe in the tire circumferential direction to a normal-linedirection of the tread surface is set in a range of 10 to 45 degrees,and the amplitude of the bent portions of the sipe in the tire radialdirection is set in a range of 0.5 to 5.0 mm.
 3. A pneumatic tire wherea plurality of longitudinal grooves extending in a tire circumferentialdirection and a plurality of lateral grooves extending in a tirewidthwise direction are provided in a tread portion, a plurality ofblocks are defined by these longitudinal and lateral grooves, and aplurality of sipes extending in the tire widthwise direction areprovided to each of the blocks, wherein, with regard to each of thesipes, a zigzag shape with an amplitude in the tire circumferentialdirection is formed on a tread surface, bent portions ranging in thetire widthwise direction while bent in the tire circumferentialdirection are formed inside the block at at least two positions in thetire radial direction, and a zigzag shape with an amplitude in the tireradial direction is formed in each of the bent portions, and wherein,while the amplitude of the sipe in the tire circumferential direction isset constant, a tilt angle of the sipe in the tire circumferentialdirection to a normal-line direction of the tread surface is set smallerat a portion closer to the bottom of the sipe than that in a portioncloser to the tread surface, and the amplitude of the bent portions inthe tire radial direction is set larger at a portion closer to thebottom of the sipe than that in a portion closer to the tread surface.4. The pneumatic tire according to claim 3, wherein tilt angles of thesipe in the tire circumferential direction to the normal-line directionof the tread surface are not smaller than 30 degrees, but not largerthan 45 degrees in the portion closest to the tread surface, and notsmaller than 15 degrees, but smaller than 30 degrees in the portionclosest to the sipe bottom, respectively.
 5. The pneumatic tireaccording to any one of claims 3 and 4, wherein, the amplitude of thebent portions in the tire radial direction is set not less than 0.5 mmin the portion closest to the tread surface, and is set not more than3.5 mm in the portion closest to the sipe bottom.
 6. A pneumatic tirewhere a plurality of longitudinal grooves extending in a tirecircumferential direction and a plurality of lateral grooves extendingin a tire widthwise direction are provided in a tread portion, aplurality of blocks are defined by these longitudinal and lateralgrooves, and a plurality of sipes extending in the tire widthwisedirection are provided to each of the blocks, wherein, with regard toeach of the sipes, a zigzag shape with an amplitude in the tirecircumferential direction is formed on a tread surface, bent portionsranging in the tire widthwise direction while bent in the tirecircumferential direction are formed inside the block at at least twopositions in the tire radial direction, and a zigzag shape with anamplitude in the tire radial direction is formed in each of the bentportions, and wherein, while intervals between the bent portions in thetire radial direction are set uniform, the amplitude in the tirecircumferential direction is set smaller in a portion closer to thebottom of the sipe.
 7. The pneumatic tire according to claim 6, wherein,if it is assumed that there are a reference line passing through aposition representing an outer limit of the amplitude of the sipe on atread surface and extending in a normal-line direction of the treadsurface, and an auxiliary line defining, along the depth direction ofthe sipe, the amplitude of the sipe in the tire circumferentialdirection, a distance at the sipe bottom between the reference line andthe auxiliary line is set more than 0% and not more than 50% of theamplitude of the sipe in the tire circumferential direction on the treadsurface.
 8. The pneumatic tire according to claim 6, wherein, if it isassumed that there are a reference line passing through a positionrepresenting an outer limit of the amplitude of the sipe on the treadsurface and extending in a normal-line direction of the tread surface,an upper auxiliary line defining, in the upper portion of the sipe, theamplitude of the sipe in the tire circumferential direction, and a lowerauxiliary line defining, in the lower portion of the sipe, the amplitudeof the sipe in the tire circumferential direction, tilt anglesrespectively of the upper and lower auxiliary lines to the referenceline are set different from each other, and the tilt angle of the lowerauxiliary line is set larger than that of the upper auxiliary line. 9.The pneumatic tire according to claim 8, wherein a distance at the sipebottom between the reference line and the upper auxiliary line is setmore than 0% and not more than 25% of the amplitude of the sipe in thetire circumferential direction on the tread surface, and a distance atthe sipe bottom between the reference line and the lower auxiliary lineis set not less than 25% and not more than 50% of the amplitude of thesipe in the tire circumferential direction on the tread surface.
 10. Thepneumatic tire according to any one of claims 6 to 9, wherein, oncondition that the pneumatic tire is provided on a superficial portionof the block with a plurality of shallow grooves, whose depth is in arange of 0.1 to 1.0 mm, and which are shallower than the sipes, avertical portion extending in a normal-line direction of the treadsurface is provided to the sipe in a section where the sipe joins to thetread surface.
 11. The pneumatic tire according to claim 10, wherein aheight of the vertical portion of the sipe is set not less than thedepth of the shallow groove.